The technique described hereinafter has been studied by the inventors to study and accomplish the present invention. This technique is summarized as follows.
A process for the production of a semiconductor device comprises forming a resist layer as a resist pattern on, e.g., a silicon semiconductor wafer (semiconductor substrate), subjecting the silicon semiconductor wafer to treatment such as injection of impurity ion and etching using the resist layer as a mask, and then removing unnecessary resist layer by means of an ashing apparatus (carbonization apparatus) to form a circuit element or wiring.
In recent years, with the enhancement of integration or function of semiconductor devices, the development of devices comprising memory and logic mounted thereon in admixture has been under way.
In the design of such a device, it is necessary that two gate oxide layers having different thicknesses be formed in the same chip.
To this end, the first gate oxide layer thus formed is patterned with a resist layer, etched using the resist layer as a mask, and then freed of the resist layer.
In this manner, the gate oxide layer is left on the semiconductor substrate corresponding to the patterning resist layer.
Thereafter, a second gate oxide layer is formed on the remaining gate oxide layer to form two gate oxide layers. The second gate oxide layer can be patterned in the same manner as in the patterning of the first gate oxide layer.
Various ashing apparatus are described in, e.g., the October 1994, issue of "Monthly Semiconductor World", Press Journal, Sep. 20, 1994, pp. 98-101.
However, the plasma ashing technique is disadvantageous in that since a resist layer is formed directly on a gate oxide layer, and then removed after patterning, damage or stain on the gate oxide layer occurring at the step of removing the resist layer has a critical effect on the device properties.
In order to remove the resist layer not causing damage or stain on the gate oxide layer, a wet process in which a chemical liquid is used or a dry process in which an ashing apparatus which hardly gives damage is used.
However, the foregoing wet process is disadvantageous in that the use of a solvent worsens the working atmosphere. On the other hand, the foregoing dry process using an ashing apparatus is disadvantageous in that plasma treatment gives damage on the gate oxide layer.